1. Field of the Invention
The invention relates to a ceramic heat barrier coating having low thermal conductivity, a process for depositing such a ceramic coating, and to metal articles protected by the coating. The invention is particularly applicable to the protection of hot superalloy components of turbomachines, such as the turbine blades or diffusers.
2. Summary of the Prior Art
The manufacturers of turboengines, whether for use on land or in aeronautics, face constant demands to increase engine efficiency and reduce fuel consumption. One way of addressing these demands is to increase the burnt gas temperature at the turbine inlet. However, this approach is limited by the ability of the turbine components, such as the diffusers and moving blades of the high pressures stages, to withstand high temperatures. Refractory metallic materials known as superalloys have been developed to make such components. These superalloys, which are nickel or cobalt or iron based, give the component mechanical strength at high temperature (creep resistance). The maximum temperature at which these superalloys can be used is 1100.degree. C., which is well below the temperature, typically 1600.degree. C., of the burnt gases at the turbine inlet. The blades and diffusers are therefore provided with internal cavities and are cooled by convection by the introduction of air into these internal cavities taken at a temperature of 600.degree. C. from the compressor stages. Some of this cooling air flowing in the internal channels of the components discharges through ventilation apertures in the wall to form a film of cool air between the surface of the component and the hot turbine gases. To obtain significant temperature gains at the turbine inlet it is known to deposit a heat barrier coating on the components.
Heat barrier technology consists of coating the components with a thin insulating ceramic layer varying in thickness from a few tens of micrometres to a few millimetres. The ceramic layer typically consists of zirconia stabilised with yttrium and has the advantages of low thermal conductivity and the good chemical stability necessary in the severe conditions experienced during turbine operation. A bonding sublayer of an aluminoforming metal alloy can be interposed between the superalloy and the ceramic layer and serves to boost the adhesion of the ceramic layer while protecting the substrate from oxidation.
However, the application of a ceramic coating to a metal article poses the problem of differential expansion of the metal and the ceramic during thermal cycling. The thermal expansion coefficient of zirconia-based ceramics, although relatively high, is still appreciably below that of metals. The microstructure of the coating must therefore be controlled so as to be able to withstand, without flaking, the heat deformations caused by the metal substrate.
Heat spraying and physical deposition in the vapour phase of an electron beam, called EB-PVD (electron beam physical vapour deposition) for short, are the two industrial processes used to deposit the heat barriers. For application to the aerodynamic part of the blades and diffusers the EB-PVD method is preferred to heat spraying, mainly because it gives a coating with a better surface texture and reduces obstruction of the ventilation apertures. Also, the EB-PVD process helps to provide the layer with a microstructure in the form of microcolumns perpendicular to the article surface. The microstructure enables the coating to deal with thermal and mechanical deformations in the plane of the substrate. For this reason EB-PVD heat barriers have a thermomechanical fatigue life which is considered to be better than that of plasma-sprayed ceramic layers.
In vapour deposition processes the coating is the result of vapour condensing on the article to be covered. There are two categories of vapour phase processes--physical processes (PVD) and chemical processes (CVD). In physical vapour phase processes the coating vapour is produced by vaporization of a solid material, also called the target. Vaporization can be produced by evaporation caused by a heat source or by cathodic atomization, a process in which the material is atomized by ionic bombardment of the target. In chemical vapour phase processes the coating vapour is the result of a chemical reaction between the gaseous components, which occurs either in the vapour phase or at the coating/gas interface. The vapour phase deposition processes are carried out in a controlled atmosphere to prevent contamination or pollution of the deposits by reaction with unwanted gas components. To this end, the deposition chamber is preliminarily exhausted to a secondary vacuum (between 10.sup.-6 Torr and 10.sup.-4 Torr) and baked. An inert or reactive working gas can be introduced in a controlled manner during deposition.
The evaporation of refractory and ceramic materials requires intense heating means. Accordingly, electron beam heating is used. The ceramic material to be evaporated is in the form of sintered bars whose surface is swept by a focused electron beam. Some of the kinetic energy of the beam is converted into heat on the bar surface. A particular feature of the EB-PVD process is that the working pressure is reduced so as to facilitate evaporation of the bars and the transfer of coating vapour from the target to the substrate. Also, electron guns require pressures of less than 10.sup.-4 Torr if they are to operate (arcing problems) which means that the electron gun must be pumped separately from the pumping of the chamber.
During the EB-PVD deposition of heat barriers the articles are heated to a high temperature of around 1000.degree. C. by radiant heating of the bars. The surface temperature thereof is estimated to be 3500.degree. C. At this temperature some of the zirconia molecules from the bar surface are dissociated in the reaction: EQU ZrO.sub.2 -&gt;ZrO+1/2O.sub.2
Some of the oxygen thus dissociated from the zirconium oxide molecules is lost as a result of the pumping of the chamber, with the consequence that the zirconia deposits are rendered substoichiometric (oxygen depleted). This effect can be countered by the introduction of an oxygen-rich gas (typically a mixture of argon and oxygen) at a pressure of a few milli-Torr into the chamber during the deposition. The effect can also be corrected ex-situ when no reactive gas is introduced into the chamber during deposition. The stoichiometry of the coating is then restored by subjecting the coated articles to a simple annealing in air at a temperature of 700.degree. C. for 1 hour. The introduction of oxygen into the EB-PVD chamber also helps to preoxidise the articles in situ before the ceramic deposition. The alumina film thus formed on the surface of the bonding sublayer provides satisfactory adhesion of the ceramic layer. In the industrial EB-PVD process only those article surfaces facing the vaporization source are coated. To cover an article of a complex geometrical shape, such as a rotor blade or a diffuser, the article must be rotated in the flow of coating vapour.
EB-PVD ceramic layers may have undeniable advantages for use on turbine blades, but they suffer from the major disadvantage of a thermal conductivity (typically from 1.4 to 1.9 W/mK) which is twice that of plasma sprayed heat barriers (from 0.5 to 0.9 W/mK). This difference in thermal conductivity is associated with the morphology of the deposits. The ceramic microcolumns perpendicular to the article surface which are found in EB-PVD depositions offer little hindrance to heat transfer by conduction and by radiation, whereas plasma sprayed depositions have a network of micro cracks which extend substantially parallel to the plane of the deposit, usually in the form of incomplete joints between the ceramic droplets which are crushed in the spraying. These micro cracks are much more effective in preventing heat conduction through the deposit. The insulation provided by a ceramic layer is proportional to its conductivity and thickness. For a given insulation level, halving the thermal conductivity of the ceramic layer would enable the coating thickness to be approximately halved--a considerable advantage when used on rotor blades subjected to centrifugal force.
WO 96/11288 describes a composite laminated heat barrier coating consisting of a stack of nanometric layers of a thickness between 0.001 and 1 micrometre and of a different nature (typically zirconia/alumina). The reduced thermal conductivity associated with such a structure is attributed to the dispersion of the phonons at the interfaces between the layers, the phonons being largely responsible for conductive transfer in dielectrics. The aforementioned document describes multilayer coatings of small thickness of the order of 4 to 5 micrometres which have a thermal conductivity half the value calculated from a law of the mixture. The reduction in thermal conductivity provided by this coating results from the creation of interfaces between two layers of different kinds. However, a sandwich structure of this kind consisting of nanometric layers suffers from thermal instability. During long spells at the high temperatures (about 1100.degree. C.) characteristic of the operating conditions experienced in turbines the fine layers may interdiffuse and homogenize the material--i.e., the interfaces responsible for reducing the thermal conductivity disappear.
WO 93/18199 and EP 0705912 disclose a heat barrier comprising a ceramic coating which consists of a number of layers of different structure. Adjacent layers have a different structure from one another in order to be able to produce an interface between each layer. The columnar morphology of the coating through its thickness is retained, this being considered to be a basic property for withstanding thermal cycling. In this coating the multilayer structure is obtained by intermittent ionic bombardment of the layer surface in conjunction with vapour condensation. The ionic bombardment is produced by polarising the article at a high negative voltage so that the article becomes the cathode of a gas discharge. The effect of the intermittent bombardment on the morphology of the resulting ceramic layer leads to the creation of relatively dense ceramic layers. However, a layer of this kind is not suitable for retaining a low thermal conductivity since heat ageing leads of course to densification of the columns and reduces the density spread between the various layers. Also, the association of a high voltage with the high temperatures (1000.degree. C.) required for EB-PVD deposition greatly complicates industrial implementation of the method.
As a rule, heat barriers having a laminated microcomposite structure (more commonly called a multilayer structure)--i.e., a microstructure based on the presence of interfaces to increase the resistance to heat flow--are unsuitable for high-temperature use because of the instability of the interfaces in operation. Because of diffusion at high temperatures an interface between two materials having a different composition or structure from one another breaks down in a graduated zone. This implies the disappearance of the interface and its associated heat resistance.